Or you could just, Place a copy of the database backup on the app's volume mayhaps
Seems the need/request for alternate backup strategies has been out there for a long time (seen a post from 2020 asking for it -> https://forum.cloudron.io/topic/2503/nextcloud-backup-without-data -> kinda left at an unsatisfactory resolve; settling for rsync with some hackarounds). My backup today with an incremental btrfs snapshot just took a couple minutes to transfer to the USB drive - and in fact, I later realized I didn't need it at all! The prior snapshot already existed (as it has to increment from one shot to the next, the previous snapshot remains in place), so I could've just browsed that - even if I lost all my files.
Regardless, I found my friend in this at the "Terminal" for the Nextcloud app:
PGPASSWORD=${CLOUDRON_POSTGRESQL_PASSWORD} pg_dump -h postgresql -U ${CLOUDRON_POSTGRESQL_USERNAME} -d ${CLOUDRON_POSTGRESQL_DATABASE} -f /your/backup/location/database.sql
Takes just a couple seconds, bang-bang and done, database secured. Would be nice to have a button to "export database" or the like, for alternate backup modes like this. I don't expect to be able to one-click restore it without support for this btrfs mechanism, but I know the data is there - and in case of disaster, there's a lot bigger things to worry about.
The current backup mechanism might work for non-data-centric apps, but for Nextcloud, it's enormous and needs outside-the-box thinking for efficiently managing terabytes of data in backup.
(Not going to use it, but I think I also located the database itself, at /home/yellowtent/platformdata/postgresql
- not something to be directly backed-up or manipulated of course )